From language/setbagmix

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Returns True if $a contains $b as an element, else False. More information, Wikipedia definition.

From language/operators

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multi sub infix:<(cont)>($a,$b --> Bool:D)

Membership operator.

Returns True if $a contains $b as an element.

say (1,2,3) (cont) 2; # OUTPUT: «True␤» 
say (1, 2, 3) ∋ 4;    # OUTPUT: «False␤» 

Since release 2020.05, ∍ is an alias for this operator.